1. Field of the Invention
The present invention relates to a System and method for manufacturing a single-crystal ingot by means of the Czochralski technique (herein after referred to simply as the xe2x80x9cCZ techniquexe2x80x9d) and more particularly, to a system and method suitable for manufacturing a perfect-crystal silicon wafer of good quality.
2. Related Art
Crystal defects which arise in a CZ single-crystal silicon ingot during the course of growth of the ingot according to the CZ technique adversely affect the reliability of a gate oxide film of an MOS device or the leakage characteristic of a p-n junction. For these reasons, the crystal defects must be minimized. It has already been pointed out that appropriate control of a ratio V/G of a pull rate V of a single-crystal ingot which is being pulled in a furnace (hereinafter simply called a xe2x80x9csingle-crystal pulled ingotxe2x80x9d) to the temperature gradient G of the single-crystal pulled ingot is effective for controlling generation of crystal defects (see, for example, Japanese Patent Laid-Open No. 337490/1996).
The inventors who contrived the invention described in publication No. 337490/1996 produced a perfect crystal by appropriate control of the ratio V/G from 1300xc2x0 C. to the melting point of silicon [see the 54th Technical Meeting of the Japan Society of Applied Physics (held for four days from Sep. 27 to 30, 1993)]. However, as described in the collection of proceedings of the 54th Technical Meeting of the Japan Society of Applied Physics (Proceeding No. 1, p. 303, 29a-HA-7) and Japanese Patent Laid-Open No. 330316/1996, the applicant of the present patent application has reported that a silicon of a perfect crystal is produced under more appropriate conditions by adequate control of the ratio V/G from 1350xc2x0 C. to the melting point of silicon.
In order to produce a perfect crystal through growth of a CZ single-crystal ingot, the pull rate V and the temperature gradient G must be controlled to a high degree of accuracy.
Even if the growth rate V is controlled constantly, the temperature gradient G changes incessantly during the course of growth of a crystal, and the value of the ratio V/G also changes so as to follow the change in the temperature gradient G. Eventually, a complete crystal region cannot be formed at good yield in the direction of crystal growth.
To overcome this problem, according to the related art (Japanese Patent Laid-Open No. 268794/1996), an internal temperature distribution of the single-crystal ingot is determined through calculation of temperature distribution of the entire furnace by use of heat transfer calculation. The heat radiated from a melt is shielded and/or reflected on the basis of the thus-calculated temperature distribution of the furnace, to thereby control the internal temperature distribution of a single-crystal ingot. Through such a control, the growth rate V is controlled such that the value of the ratio V/G (mm2/xc2x0 C.xc3x97min.) approaches a target value, and the temperature gradient G is also controlled by means of shielding and/or reflecting the heat radiated from the melt.
According to the related art, the shielding of the heat radiated from the melt is controlled by changing the position of a heat-shielding member which is a standard component of a recent system for manufacturing a CZ single-crystal ingot. The recent system has a mechanism for vertically moving a radiation shield (i.e., a heat-shielding member) in order to control the degree to which the heat radiated from a melt to a single-crystal ingot is shielded. According to the background art, a radiation reflection member of high reflectivity is placed at a position above a melt, and the amount of heat radiated is controlled by means of controlling the angle of the radiation reflection member.
Further, according to the related art, a G-calculator is used for determining the temperature gradient G at temperatures ranging up to 1300xc2x0 C. with respect to a crystal longitudinal axis. Various data sets, such as data pertaining to the position and angle of the radiation shield, are input to the G-calculator, thus calculating the temperature gradient G. On the basis of the temperature gradient G determined by the G-calculator and the growth rate V of a single-crystal, a V/G controller calculates the ratio V/G and controls the growth rate V such that the calculation result matches a predetermined V/G value, thereby regulating the position and angle of the radiation shield. Finally, the V/G ratio is controlled during the entire process.
However, in a case where the ratio V/G is controlled by changing the position and angle of the heat-shielding member, as in the case of the related art (i.e., Japanese Patent Laid-Open No. 268794/1996), additional members or mechanisms for actuating the heat-shielding member are required, thus introducing the disadvantages of an increase in the number of components and complicated control. More Specifically, under the method described in Japanese Patent Laid-Open No. 268794/1996, the heat radiated from a melt is shielded or reflected through use of various control devices such as those mentioned previously, thereby controlling a temperature gradient in the vicinity of the interface between solid and melt. An additional mechanism for controlling the position of the heat-shielding member and the angle of a reflecting member is required, thus greatly complicating the control of the ratio V/G.
Further, according to the related art, the temperature gradient G used for calculating the ratio V/G is computed basically through inference by use of simulation of a temperature. Therefore, the ratio V/G is not controlled so as to completely reflect the internal state of the furnace. Even if the growth rate V is constant, there may arise a case where the temperature gradient G is not actually controlled. In such a case, a perfect crystal cannot be produced with good yield in the direction of crystal growth.
The present invention has been conceived against the foregoing backdrop, and is aimed at providing a system and method of producing a perfect crystal with good reproducibility through growth of a single-crystal ingot. Further, the present invention provides a mechanism or method for use with a system of manufacturing a single-crystal ingot by means of the CZ technique, the mechanism or method being able to control a V/G ratio without involvement of a change in the position of a heat-shielding member.
To solve the drawbacks of the background art, there are provided a system and method of manufacturing a single-crystal ingot by means of the CZ technique according to the present invention, wherein a heat-shielding member is in principle placed in a fixed position, and the requirements for pulling a single-crystal ingot are controlled by means of measuring the distance between the bottom of the heat-shielding member and the level of molten raw material and feeding back the thus-measured distance.
In a system for manufacturing a single-crystal ingot by means of the CZ technique (hereinafter referred to simply as a xe2x80x9csystemxe2x80x9d), a crucible for storing silicon melt is moved upward so as to compensate for a drop in the level of the molten raw material stemming from a reduction in the level of molten raw material caused by pulling of a single-crystal ingot. The distance between the bottom of the heat-shielding member and the level of the molten raw material remains essentially constant. However, there is a case where a minute change arises in the distance, for reasons of a discrepancy between the rate at which the crucible is moved vertically and reduction in the level of the molten raw material, improper control of vertical movement of the crucible, deformation of the heat-shielding member caused by the internal heat of the furnace, or a change in the state of the level of the molten raw material, such as ripples. The inventors have found that even a change in the distance between the bottom of the heat-shielding member and the level of the molten raw material is on the order of millimeters, and that such a change cannot be disregarded in the process for producing a perfect crystal, thus leading to completion of the present invention.
Vertical movement of the heat-shielding member have been practiced from a macroscopic viewpoint. However, minute control of vertical movement of the heat-shielding member has not been effected, and a necessity for such minute control has not yet arisen. Particularly, changing the controlling conditions for pulling a single-crystal ingot in association with a minute change of the order of millimeters in the distance between the lower end of the heat-shielding member and the level of molten raw material has not been effected thus far. The present invention has a technical significance in that, in the course of a process for producing a perfect crystal, a minute change of the order of millimeters in the distance between the lower end of the heat-shielding member and the level of the molten raw material is taken as a major factor for determining the requirements for pulling a single-crystal ingot.
More specifically, the present invention provides a system for manufacturing a single-crystal ingot by means of the Czochralski technique (hereinafter referred to simply as the xe2x80x9cCZ techniquexe2x80x9d).
(1) A system for manufacturing a single-crystal ingot by pulling a single-crystal ingot from molten raw material which has a heat-shielding member disposed so as to surround the single-crystal ingot being pulled (hereinafter referred to simply as a xe2x80x9csingle-crystal pulled ingotxe2x80x9d) for controlling the amount of heat applied to the single-crystal pulled ingot, the system comprising:
measurement means for measuring the distance between the bottom of the heat-shielding member and the level of the molten raw material; and
a controller for controlling the requirements of pulling a single-crystal ingot on the basis of the distance measured by the measurement means.
(2) Preferably, the system comprises a cooler for cooling a portion of the single-crystal pulled ingot.
(3) Preferably, the system corresponds to a system for manufacturing a single-crystal silicon ingot, and the requirements of pulling a single-crystal silicon ingot correspond to any factor selected from the group comprising the amount of heat applied to silicon melt, the level of silicon melt, and the pull rate of a single-crystal silicon ingot (hereinafter referred to as a xe2x80x9csingle-crystal silicon pulled ingotxe2x80x9d).
(4) Preferably, a perfect crystal silicon wafer is produced by means of slicing the single-crystal silicon ingot.
Further, the present invention provides the following method of manufacturing a single-crystal silicon by use of the CZ technique.
(5) A method of manufacturing a single-crystal ingot by pulling a single-crystal ingot from molten raw material, the method comprising the steps of:
measuring the distance between the level of molten raw material and the bottom of a heat-shielding member disposed so as to surround the single-crystal ingot being pulled (hereinafter referred to simply as a xe2x80x9csingle-crystal pulled silicon ingotxe2x80x9d) for controlling the amount of heat applied to the single-crystal pulled ingot; and
controlling any factor for pulling a single-crystal silicon ingot selected from the group comprising the amount of heat applied to silicon melt, the level of silicon melt, and the pull rate of a single-crystal silicon ingot, thereby controlling the temperature gradient of area G1 of the single-crystal pulled silicon ingot.
(6) The temperature gradient of area G1 of the single-crystal silicon ingot is increased by means of cooling a lower portion of the single-crystal pulled silicon ingot, thereby increasing the pull rate of the single-crystal pulled silicon ingot.
(7) An upper portion of the single-crystal pulled silicon ingot is cooled, thereby promoting dissipation of internal conductive heat in the direction in which the ingot is pulled. Accordingly, a difference of a temperature gradient between the center of area G1 of the single-crystal pulled silicon ingot and the surface of area G1 is reduced, thus promoting formation of a perfect crystal in the direction of crystal growth. [Definitions of Terms]
The expression xe2x80x9cheat applied to the single-crystal pulled ingotxe2x80x9d corresponds to heat supplied to the single-crystal pulled ingot from surroundings thereof, such as heat radiated from the level of melt or heat emitted from a heater.
The expression xe2x80x9cheat-shielding memberxe2x80x9d signifies a heat-shielding member disposed in a system for manufacturing a single-crystal ingot by use of the CZ technique. The heat-shielding member is usually disposed so as to surround a single-crystal ingot being pulled from a melt, thus controlling the amount of heat radiated from the melt or the amount of heat emitted from a heater. The heat-shielding member controls the amount of radiated heat and rectifies the flow of inactive gas introduced into a CZ furnace. Therefore, the heat-shielding member is also called a gas rectification cylinder.
The expression xe2x80x9cmeasurement means for measuring the distance between the bottom of the heat-shielding member and the level of molten raw materialxe2x80x9d may signify any type of means which can accurately measure the distance between the bottom of the heat-shielding member and the level of the molten raw material. Preferably, there is employed a melt level detector as described in, for example, Japanese Patent application No. 071149/1999. As a matter of course, the distance from a point of measurement to the level of melt is determined through use of a conventional melt level sensor, and the temperature gradient G is inferred from the thus-determined distance, to a certain extent. However, the position of the lower end of the heat-shielding member changes during the course of pulling of the single-crystal ingot. Therefore, use of a sensor capable of accurately measuring the distance between the lower end of the heat-shielding member and the level of melt, such as that described in Japanese Patent application No. 071149/1999, is preferable.
The symbol xe2x80x9cG1xe2x80x9d designates a longitudinal temperature gradient (xc2x0 C./mm) of a temperature region (a temperature region around the range from the temperature of the interface between solid and melt to 1350xc2x0 C.) at which the pattern of a grown-in defect is determined. The symbol xe2x80x9cG2xe2x80x9d designates a longitudinal temperature gradient (xc2x0 C./mm) of a temperature region at which voids are formed (a temperature region in the vicinity of 1120xc2x0 C.) The longitudinal temperature gradient G1 and the longitudinal temperature gradient G2 are concepts important for manufacturing a commercially valuable silicon wafer.
The expression xe2x80x9cperfect crystalxe2x80x9d usually corresponds to a crystal which is free of crystal defects, such as voids or dislocation clusters, and is often called none defects crystal. Such a perfect crystal contains no grown-in defects, such as voids, and no oxide precipitates. However, the perfect crystal contains oxide precipitate nuclei which would serve as the basis for formation of oxide precipitates. Hence, in a case where a perfect crystal silicon wafer sliced from a perfect crystal ingot is subjected to heat treatment, oxide precipitates arise in the wafer.
A xe2x80x9ccoolerxe2x80x9d for cooling a single-crystal pulled ingot may be embodied by any type of cooler, so long as the cooler cools a predetermined portion of the single-crystal pulled ingot. In view of reliable cooling of a specific location, a cooler comprising a piping system through which cooling water circulates or a heat pipe is desirably adopted (e.g., those described in Japanese Patent application Nos. 094695/1999 and 094697/1999).
The concept of the term xe2x80x9ctemperature gradientxe2x80x9d implies the internal temperature gradient of the single-crystal pulled ingot actually measured by the temperature sensor and the internal temperature gradient of a coagulation section which are theoretically or empirically calculated from the actually-measured surface temperature of the ingot, as well as the temperature gradient of a single-crystal pulled ingot as actually measured by a temperatures sensor.
According to the method of the present invention, a ratio V/G is determined on the basis of a temperature gradient which is calculated from the distance between the level of melt and the bottom of the heat-shielding member as measured through use of a melt level sensor or a like device, On the basis of the thus-determined V/G, the temperature of a heater and the level of melt are controlled. The V/G ratio suitable for producing a crystal can be obtained according to a substance which is an object of production, as required. Therefore, it is evident that application of the method and system according to the present invention is not limited to silicon.